The electrolysis of alkali chloride brines, for instance of sodium chloride brine for the production of chlorine and caustic soda, is often carried out with titanium or other valve metal-based anodes activated with a superficial layer of ruthenium dioxide (RuO2), which has the property of lowering the overvoltage of anodic chlorine evolution reaction. A typical formulation of catalyst for chlorine evolution consists of a RuO2 and TiO2 mixture, which has a sufficiently reduced anodic chlorine evolution overvoltage. Beside the need for resorting to very high ruthenium loadings to obtain a satisfactory lifetime at the usual process conditions, such formulation has the disadvantage of a reduced overvoltage of the anodic oxygen evolution reaction. This causes the concurrent anodic oxygen evolution reaction to be not effectively inhibited, so that product chlorine presents an oxygen content which is too high for some uses.
The same considerations apply for formulations based on RuO2 mixed with SnO2, or for ternary mixtures of ruthenium, titanium and tin oxides. In general, catalysts capable of sufficiently lowering the overvoltage of the chlorine evolution reaction, so as to guarantee an acceptable energy efficiency, tend to have the same effect on the concurrent oxygen evolution reaction, giving rise to a product of unsuitable purity. A known example in this regard is given by palladium-containing catalyst formulations, which are capable of carrying out chlorine evolution at sensibly reduced potentials, but with a much higher content of oxygen in the chlorine, in addition to their limited lifetime.
A partial improvement in terms of duration and of oxygen evolution inhibition is obtainable by adding a formulation of RuO2 mixed with SnO2 with a certain amount of a second noble metal selected between iridium and platinum, as described in the prior art. The activity of this electrode—in terms of cell voltage and consequently of energy consumption—is nevertheless not yet ideal for the economics of a large scale industrial production.
It becomes therefore necessary to identify a catalyst formulation for an electrode suitable for functioning as a chlorine-evolving anode in industrial electrolysis cells presenting characteristics of improved anodic chlorine evolution potential jointly with an adequate purity of product chlorine.